Since the discovery that conjugated polymers can be made to conduct electricity through doping, research has been extended in the field of conducting polymer films. Polymers have been made as conducting links of organic monomers having defined chemical structures. Polyaniline can be made as a conducting polymer of aniline monomers. Polyaniline is a unique conjugated polymer in that polyaniline can be tailored for specific applications through a non-redox acid and base doping process. Polyaniline has been studied for electronic and optical applications, such as lightweight battery electrodes, electromagnetic shielding devices, anticorrosion coatings, and sensors. Unlike other conjugated polymers, polyaniline has a simple and reversible acid doping and base dedoping chemistry enabling control over properties of the polyaniline, such as density, solubility, conductivity, and optical absorption. One-dimensional polyaniline nanostructures, including nanowires, nanorods, and nanotubes possess low-dimensional sizes and organic conduction. The electrically conductive form of polyaniline is known as emeraldine having an oxidation state which, when doped with an acid, protonates the imine nitrogens on the polymer backbone and induces charge carriers. The conductivity of polyaniline increases with doping from the undoped insulating emeraldine base form, σ<10−10 S/cm, to the fully doped, conducting emeraldine salt form, σ>1 S/cm. Dopants can be added in any desired quantity until all imine nitrogens, that is half of the total nitrogens, are doped, by controlling the pH of the dopant acid solution. Dopants can be removed by interacting the emeraldine salt form with common bases such as ammonium hydroxide.
Conducting polymers can be used in sensors having optical, electrochemical and conducting properties. Conducting polymers are unique by changing properties when chemically treated with oxidizing or reducing agents. After chemical treatment with protonating, deprotonating, oxidizing or reducing agents, the conducting polymer polyaniline can reversibly change from an initially electrically insulating state to a conducting state. This transition can be used in such applications as optical sensors, chemical sensors, and biosensors. Conducting polymers include polyaniline, polypyrrole, polythiophene, and their derivatives. Polyaniline is a conducting polymer that is environmentally stable and can react with chemical species at room temperature. As such, polyaniline may be suitable for gas sensing applications using processes that create a uniform thin film of the polyaniline. This thin film may then react with protonating and deprotonating agents to create a conduction pathway that can easily be measured.
The conductivity depends on both the ability to transport charge carriers along the polymer backbone and the ability of the carriers to hop between polymer chains through interpolymer conduction. Any interactions with polyaniline that will alter either of these conduction processes will affect the overall conductivity. This is the underlying chemical property enabling polyaniline to be used as the selective layer in a chemical vapor sensor, such as, a resistance detector generally known as a chemiresistor. Due to room temperature sensitivity, the ease of deposition onto a wide variety of sensor substrates and due to the various structures, conducting polymers are potential materials for sensor applications. A polymer chemiresistor would typically consist of a substrate, electrodes, and a conducting polymer selective thin film. Changes in conductivity of the polymer film upon exposure to chemical vapors can be readily monitored with an ohmmeter or electrometer. Polyaniline sensor research has focused on changing the polymer structure to facilitate interaction between vapor molecules and the polymer either by modifying the polymer backbone or the interchain connections. However, poor diffusion can readily outweigh any improvements made to the polymer chains because most of the material other than the limited number of surface sites, is not available for interacting with a chemical vapor, thus degrading sensitivity. One way to enhance diffusion is to reduce film thickness, such as producing monolayers of conventional polymer materials, which leads to a trade-off between sensitivity and robustness. Coating polyaniline on porous substrates can increase the surface area, but the chemistry and physics involving polymer support and polymer electrode interfaces is not well defined for practical use.
Nanostructured polyaniline, such as nanowires, nanofibers, nanotubes, and nanorods may have sufficiently high surface area and faster diffusion rates of gas molecules into the nanostructures for use as chemical sensors with increased sensitivity, as compared to bulk polyaniline. For example, the surface area per unit mass SA of polyaniline nanofibers increases geometrically as the diameters d of the nanofibers decrease, that is SA˜1/d. Even when the thickness of an ultra-thin film is the same as the diameters of the nanofibers, the fibers may outperform a thin film because the fibers have higher surface-to-volume ratios due to their cylindrical morphology. The small diameter of the nanofibers, for example less than 500 nm, coupled with the possibility of gas approaching from all sides should give sensors with improved performance. Despite the high surface area and porosity associated with nanostructures, nanostructured polyaniline has not been used as chemical sensors. This is due to uncertain nanostructure characterization as well as the lack of reliable methods to make high quality polyaniline nanofibers, and reliable methods to coat surfaces with polyaniline nanofibers. No practical nanostructured conducting polymer sensors are available due to the lack of reliable methods for making high quality conducting polymer nanostructures in bulk quantities and the unknown properties of nanofiber characterization.
Syntheses of polyaniline nanostructures have been carried out both chemically and electrochemically by polymerizing the aniline monomers with the aid of either a hard template or a soft template. Examples of hard templates include zeolite channels, track-etched polycarbonate, nanoporous membranes, and anodized alumina. Examples of soft templates for self-assembly of functional polymers include surfactants, polyelectrolytes, or complex organic dopants, such as micelles, liquid crystals, thiolated cyclodextrins, and polyacids, that may be capable of directing the growth of polyaniline one-dimensional nanostructures with diameters smaller than 500 nm. Adding structural directing molecules such as surfactants or polyelectrolytes to the chemical polymerization bath is one way to obtain polyaniline nanostructures. When organic dopants with surfactant functionalities are used, emulsions or micelles can be formed leading to microtube, microfiber, or microrod structures. However, when polyaniline nanostructures with diameters of less than 500 nm are desired, then very complex dopants with bulky side groups are needed, such as sulfonated naphthalene derivatives, fullerenes, or dendrimers.
The formation of polyaniline nanostructures disadvantageously relies either on guidance from hard templates or self-assembled soft templates. These templates disadvantageously use complex synthetic conditions that require the removal of such templates and hence provide low yields and with poor reproducibility. Chemical methods of making polyaniline nanostructures, such as nanotubes, nanofibers, nanowires, and nanorods, disadvantageously require specific structure-directing template materials added into or applied to the polymerization bath. The synthetic conditions disadvantageously have to be carefully designed to accommodate formation and purification to obtain pure polyaniline nanostructures. These template methods are disadvantageously dependent on either a template or a specific complex chemical reagent, and post-synthetic treatments are needed to remove the reagent from the byproducts in order to recover pure nanostructured polyaniline. Therefore, developing synthetic production methods that do not rely on templates, structural directing molecules, or specific dopants is desirable, especially for scaling up to produce large quantities of nanostructured materials suitable for mass usage in chemical sensors.
Electrochemical polymerization and physical methods, such as electrospinning and mechanical stretching can produce conducting polymer nanofibers without templates, but these conducting polymer nanofiber materials can only be made on carefully prepared surfaces offering limited production scaling. Electrochemical synthesis of polyaniline has indicated that some nanofibers form naturally on a synthesis surface while the underlayer is much more compact with microfiber polymers. For the production of polyaniline nanofiber sensors in quantity, there exists a need for a practical bulk synthetic method. Despite the variety of current synthetic methods available to produce polyaniline nanostructures, there is a need for a practical synthetic method capable of making pure, uniform, and template-free polyaniline nanostructures with predetermined small diameters and in bulk quantities. Current synthetic methods are not useful in mass production of ultra-small, low-dimensional structures, such as sensors, using conductive polymer nanofibers of polyaniline. These and other disadvantages are solved or reduced using the present invention.